Add a foreground notification to the background service

Summary:
This way, the user always knows that KDE Connect is running (and what devices are connected). This is a requirement in Android Oreo.

Implements task T6218.

Reviewers: KDE Connect, nicolasfella

Subscribers: apol, albertvaka, nicolasfella

Differential Revision: https://phabricator.kde.org/D8966